This invention is related to the art of DiLitho printing and to improvements in apparatus therefor.
The DiLitho printing process was developed about 1968 by the American Newspaper Publishers Association Research Institute in an effort to help obtain for those printing plants which had large investments in continuous letterpress printing equipment some of the advantages which were then appearing for offset printing such as the ability to utilize photocomposition and its attendant cost savings. This process operates in a fashion analogous that of offset in that ink is supplied to a substantially flat oleophilic (hydrophobic) image area on a carrier preferably cylindrical, for continuous operation, while an aqueous fountain solution is supplied to hydrophilic non-image areas. However, the image is transferred as in letterpress operation directly from the inked image to the paper web. The conversion of existing continuous letterpress units is thus facilitated because the plate cylinder merely needs the DiLitho plates fastened on in place of letterpress image plates, a fountain solution application train must be added, and the inking system requires relatively minor modifications to accommodate the new ink and fountain solution applicator. This conversion is far less costly than the entire replacement of a letterpress unit with an offset unit.
Although the above advantages have led to fairly widespread commercial use of this process a number of problems remain. These are the potential for poor print quality because of inadequate ink transfer, a high volume of start-up waste because of slow initial inking of the plates (as much as 4% of the paper), susceptibility to linting requiring frequent shut downs for wash-ups, and, since the plate contacts the paper web directly, wear on the printing plate limits the number of impressions obtainable from a plate.
Certain improvements in these characteristics may be employed by running the process at higher temperatures but just as in regular offset this increases problems with ink and fountain solution mixing reducing print definition and leading to background shading and also with ink print through on the paper because of the lowered ink viscosity.